When sizing a gas regulator you must first know the
following four parameters

Inlet Pressure
(Sometimes called P in) This is important in order to select
the proper pressure rating of the valve as well as being necessary
to determine the allowable pressure drop across the valve.
Here in the United States, Pressure is commonly expressed
in Inches of Water (" wc) for low pressure applications
and Pounds per Square Inch (PSI) for higher pressure applications.
(Note: 1 PSI equals 28" wc.)

Outlet Pressure
(Sometimes called P out) This is necessary to select the proper
spring range of the regulator along with the other piece in
the puzzle for determining the allowable pressure drop. The
smaller the allowable pressure drop is the bigger the valve
must become, and vice versa. An application with a large drop
across the regulator will have a much smaller regulator size.

Flow
This coupled with the allowable pressure drop (P out minus
P in) and determines the actual size of the regulator body
and valve orifice. The flow is commonly expressed in CFH or
CFM (cubic feet per hour or minute respectively) for gaseous
applications and GPH or GPM (gallons per hour or minute respectively)
for liquid applications. When dealing with combustion or plumbing
& heating, it is also very common to speak in BTUH, which
is British Thermal Units per hour, or MBH which is thousands
of BTUs per hour. Any of these can tie your flow back to a
specific rate of gas flow.

Application
It is important to define the application with all of its
special expectations. The materials of construction in a High
Pressure Steam application would differ significantly when
compared to a Low Pressure Natural Gas application. Also,
if you're locating the regulator in the wall next to your
boss's office, noise may be a consideration. Is it indoor
or outdoor? Issues like that are worthwhile considering in
defining your needs.

The important part is to provide your supplier
or the engineer who is going to size the regulator with the
above criteria. Anyone who advises you that this information
is NOT necessary is misinformed and mistaken.